Indicators such as stroke volume (SV), cardiac output (CO), end-diastolic volume, ejection fraction, stroke volume variation (SVV), pulse pressure variation (PPV), systolic pressure variations (SPV), and plethysmographic variability index (PVI), among others, are important not only for diagnosis of disease, but also for “real-time” monitoring of preload dependence, fluid responsiveness, or volume responsiveness condition of both human and animal subjects. Few hospitals are therefore without some form of equipment to monitor one or more of these cardiac parameters. Many techniques, including invasive techniques, non-invasive techniques, and combinations thereof, are in use and even more have been proposed in the literature. References that disclose determination of hemodynamic parameters include WO02011094487 (Jian et al., filed 28 Jan. 2011) and WO 2009023713 (Derderian et al., filed 13 Aug. 2008).
One way to obtain a hemodynamic parameter is to mount a flow-measuring device on a catheter, and position the device in or near the subject's heart. Some such devices inject either a bolus of material or energy (usually heat) at an upstream position, such as in the right atrium, and determine flow based on the characteristics of the injected material or energy at a downstream position, such as in the pulmonary artery. Patents that disclose implementations of such invasive techniques (in particular, thermodilution) include: U.S. Pat. No. 4,236,527 (Newbower et al., 2 Dec. 1980); U.S. Pat. No. 4,507,974 (Yelderman, 2 Apr. 1985); U.S. Pat. No. 5,146,414 (McKown, et al., 8 Sep. 1992); and U.S. Pat. No. 5,687,733 (McKown, et al., 18 Nov. 1997). Other invasive devices are based on the known Fick technique, according to which a hemodynamic parameter is calculated as a function of oxygenation of arterial and mixed venous blood. Doppler techniques, using invasive as well as non-invasive transducers, have also been used to obtain flow rate data that can then be used to calculate a hemodynamic parameter.
One blood characteristic that can be obtained with minimal or no invasion is blood pressure. In addition to causing minimal patient trauma, blood pressure measurement technology has the added benefit of being accurate and continuous. Many systems rely on the pulse contour method (PCM), which calculates an estimate of one or more hemodynamic parameters of interest from characteristics of a blood pressure waveform. In the PCM, “Windkessel” parameters, such as characteristic impedance of the aorta, compliance, and total peripheral resistance, are often used to construct a linear or non-linear hemodynamic model of the aorta. In essence, blood flow is analogized to a flow of electrical current in a circuit in which an impedance is in series with a parallel-connected resistance and capacitance (compliance). The three required parameters of the model are usually determined either empirically, through a complex calibration process, or from compiled “anthropometric” data, i.e., data about the age, sex, height, weight, and/or other parameters of other patients or test subjects. U.S. Pat. No. 5,400,793 (Wesseling, 28 Mar. 1995) and U.S. Pat. No. 5,535,753 (Petrucelli et al., 16 Jul. 1996) disclose systems that rely on a Windkessel circuit model to determine a hemodynamic parameter. PCM-based systems can monitor hemodynamic parameters using blood pressure measurements taken using a variety of measurement apparatus, such as a finger cuff, and can do so more or less continuously. Many improvements, with varying degrees of complexity, have been proposed for improving the accuracy of the basic PCM model. The present disclosure offers an improvement over the PCM model or any other models discussed in this section.
Yet more advanced methods for calculating hemodynamic parameters are described, for example, in U.S. Pat. Nos. 7,967,757, and 8,721,556, in PCT patent application publication numbers WO2009/023713 and WO2011/094487, and in PCT International Patent Application Serial No. US2014/045538, the full contents of each of which are hereby incorporated by reference in their entireties.